[
https://issues.apache.org/jira/browse/ARROW-15520?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Antoine Pitrou updated ARROW-15520:
-----------------------------------
Summary: [C++] Unqualified format() calls are ambiguous in C++20 (was:
Unqualified format() calls are ambiguous in C++20)
> [C++] Unqualified format() calls are ambiguous in C++20
> -------------------------------------------------------
>
> Key: ARROW-15520
> URL: https://issues.apache.org/jira/browse/ARROW-15520
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Affects Versions: 7.0.0
> Environment: Visual Studio 2022 17.2 Preview 2
> Reporter: Stephan T. Lavavej
> Priority: Major
> Labels: pull-request-available
> Time Spent: 20m
> Remaining Estimate: 0h
>
> I work on MSVC's C++ Standard Library implementation, and we regularly build
> open-source projects, including Apache Arrow, with development versions of
> the MSVC toolset in order to find and fix compiler/library bugs before they
> can cause problems for our programmer-users like you. This also allows us to
> provide advance notice of breaking changes in the C++ Standard that will
> affect you, which is the case here.
> We recently implemented C++20's std::format(), followed by the C++20 Defect
> Report [P2418R2|https://wg21.link/P2418R2] "Add Support For
> std::generator-like Types To std::format" with
> [microsoft/STL#2323|https://github.com/microsoft/STL/pull/2323]. This
> prevents Apache Arrow from compiling with the latest C++ Standard mode
> enabled, because MakeTimeFormatter() in diff.cc contains unqualified calls:
> [https://github.com/apache/arrow/blob/56e270fda7f5647a157acd1e428d9735d6399881/cpp/src/arrow/array/diff.cc#L636-L681]
> (The issue involves Argument-Dependent Lookup. The using-declaration `using
> arrow_vendored::date::format;` means that the following unqualified calls to
> `format()` will consider the overload in the `arrow_vendored::date`
> namespace, which is the desired overload. However, because the arguments are
> `std::chrono::duration` types, `std` is considered an "associated namespace",
> so it will also be searched for overloads. Our implementation of the chrono
> header includes the new format header (as permitted by the Standard - we do
> this because chrono types are formattable in C++20), so the `std::format`
> overload is visible. Finally, the signature change required by the P2418R2
> paper makes `std::format` a highly greedy "perfect forwarding" signature, so
> it's ambiguous with the desired `arrow_vendored::date::format` overload.)
> The full steps to reproduce are:
> # Build the microsoft/STL repo, and update the INCLUDE/LIB/etc. environment
> variables so that it can be consumed. (Or wait for Visual Studio 2022 17.2
> Preview 2 to ship, as it will contain these changes.)
> # Configure Apache Arrow with the latest C++ Standard version: cmake -G
> Ninja -S . -B build -DCMAKE_CXX_STANDARD=23 (note that C++23 must be selected
> at this time even though std::format() is C++20 - it's a long story)
> # diff.cc fails to compile with:
>
> {noformat}
> ninja: Entering directory `build'
> [13/191] Building CXX object
> src\arrow\CMakeFiles\arrow_static.dir\array\diff.cc.obj
> FAILED: src/arrow/CMakeFiles/arrow_static.dir/array/diff.cc.obj
> C:\PROGRA~1\MIB055~1\2022\Preview\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\cl.exe
> /nologo /TP -DARROW_EXPORTING -DARROW_HAVE_RUNTIME_AVX2
> -DARROW_HAVE_RUNTIME_AVX512 -DARROW_HAVE_RUNTIME_BMI2
> -DARROW_HAVE_RUNTIME_SSE4_2 -DARROW_HAVE_SSE4_2 -DARROW_STATIC
> -DARROW_WITH_TIMING_TESTS -DURI_STATIC_BUILD -D_CRT_SECURE_NO_WARNINGS
> -D_ENABLE_EXTENDED_ALIGNED_STORAGE -IC:\Temp\arrow\cpp\build\src
> -IC:\Temp\arrow\cpp\src -IC:\Temp\arrow\cpp\src\generated
> -IC:\Temp\arrow\cpp\thirdparty\flatbuffers\include
> -IC:\Temp\arrow\cpp\build\boost_ep-prefix\src\boost_ep
> -IC:\Temp\arrow\cpp\build\xsimd_ep\src\xsimd_ep-install\include
> -IC:\Temp\arrow\cpp\thirdparty\hadoop\include /DWIN32 /D_WINDOWS /GR /EHsc
> /D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING /EHsc /wd5105 /bigobj /utf-8
> /W3 /wd4800 /wd4996 /wd4065 /MD /O2 /Ob2 /DNDEBUG -std:c++latest
> /showIncludes /Fosrc\arrow\CMakeFiles\arrow_static.dir\array\diff.cc.obj
> /Fdsrc\arrow\CMakeFiles\arrow_static.dir\arrow_static.pdb /FS -c
> C:\Temp\arrow\cpp\src\arrow\array\diff.cc
> C:\Temp\arrow\cpp\src\arrow\array\diff.cc(652): error C2666:
> 'arrow_vendored::date::format': 2 overloads have similar conversions
> C:\Temp\arrow\cpp\src\arrow/vendored/datetime/date.h(6264): note: could be
> 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>
> arrow_vendored::date::format<_Elem,std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>>(const
> CharT *,const Streamable &)'
> with
> [
> _Elem=char,
> CharT=char,
>
> Streamable=std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>
> ]
> C:\GitHub\STL\out\build\x64\out\inc\format(3100): note: or
> 'std::wstring
> std::format<std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>>(const
>
> std::_Basic_format_string<wchar_t,std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>>,std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>
> &&)' [found using argument-dependent lookup]
> C:\GitHub\STL\out\build\x64\out\inc\format(3095): note: or 'std::string
> std::format<std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>>(const
>
> std::_Basic_format_string<char,std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>>,std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>
> &&)' [found using argument-dependent lookup]
> C:\Temp\arrow\cpp\src\arrow\array\diff.cc(680): note: while trying to match
> the argument list '(const _Elem *,
> std::chrono::time_point<std::chrono::system_clock,std::chrono::duration<__int64,std::nano>>)'
> with
> [
> _Elem=char
> ]
> C:\Temp\arrow\cpp\src\arrow\array\diff.cc(680): note: note: qualification
> adjustment (const/volatile) may be causing the ambiguity
> C:\Temp\arrow\cpp\src\arrow\array\diff.cc(451): note: see reference to
> function template instantiation 'arrow::Formatter
> arrow::MakeFormatterImpl::MakeTimeFormatter<arrow::TimestampType,true>(const
> std::string &)' being compiled
> [...]{noformat}
> The fix is very simple: remove the using-declaration and explicitly qualify
> each call. I will submit a pull request on GitHub for this.
>
> (Even if Apache Arrow isn't planning on using C++20 any time soon, making
> this change will make it easier for MSVC to continue validating Apache Arrow
> with the latest toolset and Standard changes, and it will remove a potential
> future headache if and when Apache Arrow does migrate to later Standard
> versions.)
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